In recent years, there is an increasing demand for low pollution, high power diesel engines. In this context, Jikkaisho 61-218769 published by the Japanese Patent Office, for example, discloses an engine in which combustion is improved by varying fuel injection characteristics based on engine speed or load, and raising fuel injection pressure to exceed 100 bars to convert fuel to fine droplets.
In such engines, however, a high pressure injection pump compresses the fuel through engine driven cams. Even if the injection pressure is sufficient at high engine speeds, therefore, the pressure at low engine speeds is only half of this value or less. Consequently, at low engine speed, conversion of fuel to fine droplets is poor, fuel does not mix properly with air, air is not used efficiently in the combustion process and more smoke is generated.
Better mixing of fuel and air can be promoted by setting up an air intake swirl in the combustion chamber. In direct injection diesel engines, however, the amount of smoke in the exhaust increases if the swirl ratio (intake air swirl speed/engine speed) is higher or lower than the optimum swirl ratio. Furthermore, the optimum swirl ratio varies with the injection pressure and is not constant.
It is therefore difficult to reduce the amount of smoke in direct injection diesel engines.